Ion mass filters and ion traps are used in mass spectrometers for atomic and chemical analysis to determine the quantity and atomic or chemical makeup of unquantified or unknown compounds. A quadrupole mass spectrometer system generally includes a source of ions, a quadrupole mass filter, an ion detector and associated electronics. A gaseous, liquid, or solid sample is ionized in the ion source and a portion of the ions created in the ion source is injected into the quadrupole mass filter. The filter rejects all ions except those with masses in a mass-to-charge ratio (mass/charge) window as determined by the system electronics. (It will be understood from the context herein where the references to mass without mentioning charge refer to the mass-to-charge ratio, as appropriate, even though charge is not specifically expressed, because the effects of fields on ions depend on the charge of the ions).
The mass window (or resolution) is usually adjusted to be about 1 atomic mass unit (AMU) or less, centered at a particular, selected mass. Because the masses of the elements making up the sample are often unknown, the system varies (scans) the range of selected masses from a starting mass number to an ending mass number to test for and to sense ions having masses within the selected mass range. The scanned mass range can be as low as one AMU and as high as thousands of AMU. The system operates either automatically or under manual control. The mass analysis of the composition of the sample is performed by rapidly scanning the DC and RF voltages, or the frequency of the RF voltage, applied to the quadrupole filter, thereby scanning through the possible ion masses and recording the abundance of each as transmitted through the mass filter.
Referring to FIG. 1, a conventional quadrupole mass spectrometer 30 includes a source of ions 32 driven by a suitable power supply 34 for ejecting ions from the opening 36 in the source 32. The source 32 of ions can be any one or more of a number of devices, including electron impact, atmospheric pressure chemical ionization, inductively coupled plasma, electrospray or the collision cell of a tandem mass spectrometer, e.g., a triple quadrupole.
While the ions may leave source 32 with a range of directions and velocities, they are traveling generally in the direction of the central axis 38 of the quadrupole mass filter 40. The central axis 38 is generally considered the Z-direction represented at 42. Ions may also have components of velocity in the directions of the X-axis and the Y-axis, respectively identified with reference numbers 44 and 46.
The quadrupole mass spectrometer 30 also may include ion optics 88 to generally focus the ions toward the quadrupole mass filter 40 and along the central axis 38. Elements of ion optics 88, which may be, for example, aperture lenses or ion guides, may have DC or RF voltages applied to them by one or more voltage supplies 56. Voltage supply 56 may be controlled and operated by a controller 58 or other apparatus. As an example of ion optics 88, a typical mass spectrometer with an atmospheric pressure ion source may have a skimmer and one or more RF ion guides before the quadrupole mass filter.
A conventional quadrupole mass filter 40 includes four conductive rods arranged with their long axes parallel to a central axis and equidistant from it. For most purposes, the cross sections of the rods are preferably hyperbolic, although rods of circular cross section (“round rods”) are common. Other rod shapes have been used, such as those having flat or concave faces. To select which ions are rejected and which are transmitted through the quadrupole mass filter 40, an adjustable voltage ±(U+V cos Ωt) is applied on adjacent rods, so that opposite rods have equal potentials and adjacent rods have equal potentials but opposite polarities. U is the DC voltage and V is the amplitude of the radio frequency (RF) voltage applied to the rods, Ω being its (angular) frequency. The electric field created within the region surrounded by the rods is a quadrupole field, resulting in a force on an ion in that region directly proportional to the distance of the ion from the central axis 38.
The mass filter 40 is driven by a suitable quadrupole voltage supply 64, which may be controlled by a suitable controller 58 such as a microprocessor programmed with control software and data sufficient to allow the quadrupole mass filter to scan over a desired ion mass range. As is known, the conventional quadrupole mass filter 40 filters out ions outside a narrow mass window and transmits ions within the window to an ion detector, which is formed by an ion collector 66 and an analyzer 68 that analyzes the collected ions. The analyzer 68 may be controlled by, may output results to, and may be a part of the controller 58.
Voltages applied to the quadrupole rods create an electric field between the poles that may change the radial energy of the ions entering the quadrupole filter 40. In quadrupole fields, ions have generally predictable characteristics. Ion motion in a quadrupole field has the form:
                                                                        ⅆ                2                            ⁢              u                                      ⅆ                              ζ                2                                              +                                    (                                                a                  u                                -                                  2                  ⁢                                      q                    u                                    ⁢                  cos                  ⁢                                                                          ⁢                  2                  ⁢                  ζ                                            )                        ⁢            u                          =        0                            [        1        ]            where u represents either the x or y direction, ζ=Ωt/2 and Ω is the frequency of the RF voltage applied to the quadrupole rods. Variables au and qu are defined as:
                              a          u                =                              a            x                    =                                    -                              a                y                                      =                                                            4                  ⁢                  eU                                                  m                  ⁢                                                                          ⁢                                      Ω                    2                                    ⁢                                      r                    0                    2                                                              ⁢                                                          ⁢              and                                                          [        2        ]                                          q          u                =                              q            x                    =                                    -                              q                y                                      =                                          2                ⁢                e                ⁢                                                                  ⁢                V                                            m                ⁢                                                                  ⁢                                  Ω                  2                                ⁢                                  r                  0                  2                                                                                        [        3        ]            In equations [2] and [3], U and V are the magnitudes of DC and RF voltages, respectively. The ion trajectories can be expressed as:
                              u          ⁡                      (            ζ            )                          =                              A            ⁢                                          ∑                                  -                  ∞                                                  +                  ∞                                            ⁢                                                          ⁢                                                c                                      2                    ⁢                    n                                                  ⁢                                  cos                  ⁡                                      (                                                                  2                        ⁢                        n                                            +                      β                                        )                                                  ⁢                ζ                                              +                      B            ⁢                                          ∑                                  -                  ∞                                                  +                  ∞                                            ⁢                                                          ⁢                                                c                                      2                    ⁢                    n                                                  ⁢                                  cos                  ⁡                                      (                                                                  2                        ⁢                        n                                            +                      β                                        )                                                  ⁢                ζ                                                                        [        4        ]            
where β is a function of au and qu, and thus, U and V. The trajectories exhibit oscillations in the x- and y-directions. For certain values of au and qu, the trajectories are periodic and will allow ions to pass through the mass filter without striking a rod; the ion trajectory or path is said to be stable. Whether a trajectory is stable in the field depends on where the ion mass is mapped into the stability diagram of FIG. 9. If the ion mass is such that the a, q values fall below the βy=0 and βx=1.0 lines, the trajectory will be stable. The stability thus depends upon the values of U and V for a given mass. (Stability also depends upon the initial position and velocity of the ion at the entrance to the quadrupole mass filter, as is known in the art.) If the U/V ratio and the value of V are such that the operation is near the very apex (βy=0, βx=1.0) of the stability diagram, only a very narrow range of ion masses will have stable trajectories. This is the principle of the quadrupole mass filter and it allows operation as a mass spectrometer by scanning the RF voltage while keeping the DC/RF voltage ratio constant.
Equation [4] indicates that ion motion in a quadrupole electric field consists of a set of secular frequencies given by
                                          ω            n                    =                                    (                                                2                  ⁢                  n                                +                β                            )                        ⁢                          Ω              2                                      ,                                  ⁢                  n          =          0                ,                  ±          1                ,                  ±          2                ,        …                            [        5        ]            If an auxiliary AC voltage is applied to the rods, creating an auxiliary AC electric field within the rods, and is tuned to one of the secular frequencies ωn, the ion will absorb energy and oscillate with increased amplitude. The ion also absorbs energy, with increase of trajectory amplitude, if the auxiliary AC voltage is at the parametric resonance frequency, βΩ (twice the lowest secular frequency).
The effectiveness of a mass spectrometer system is determined in large part by its sensitivity and selectivity, the latter usually being called resolution. Sensitivity determines how small a quantity of sample can be detected and its constituents quantified. Resolution must be sufficient for two adjacent mass peaks to be clearly separated such that their separate characteristics can be determined.
One can increase the resolution of a quadrupole mass filter by decreasing the RF/DC voltage ratio, but increasing the resolution decreases the number of ions transmitted through the mass filter 40. Even for a selected ion mass, the transmission of ions through the mass filter 40 and output from the mass filter 40 is a fraction of the ions input to the mass filter 40. With lower transmission, the amount of ions in the sample becomes more important and it may be more difficult to qualify the results for each mass peak in a spectrum as signal to noise ratios decrease. The resolution achievable depends on accuracy of the quadrupole electric field, the mass of the ion and the length of the mass filter 40, and the transmission depends on the resolution and the input conditions of the ions, i.e., on the positions and velocities of the ions as they enter the mass filter 40. Other factors affect the operation of the mass filter 40, such as fringe fields at the ends of the mass filter 40, the presence or absence of focusing elements, and the voltages that may be applied to these focusing elements. While many of these factors are understood, there is room for improvement in the resolution and sensitivity of mass filter spectrometers.
Quadrupole mass filters for use in mass spectrometers as described above have several shortcomings. For example, electrodes for quadrupole mass filter spectrometers having a unit mass resolution and reasonable ion transmission rate are fabricated with precisions on the order of tenths of microns to microns. The cost of such precision quadrupole mass spectrometers is high.
A quadrupole mass filter can be operated in an RF-only mode without a DC voltage between adjacent rods. In this design, ion radial motion is excited by bringing the ions to the boundary of the ion stability diagram. Mass separation is achieved by coupling the ion radial (x, y) motion and the axial (z-direction) motion in the exit region. A spectrum of masses is obtained by varying or scanning the RF voltage to sequentially bring the q of various masses close to a value of about 0.907, in other words near the stability boundary of the ion stability diagram on the q-axis. At that point, ions having the mass of interest acquire large radial energies that are converted by the fringing fields to increased axial energy relative to ions of other masses when exiting the quadrupole field. The increased axial energy allows those ions to pass through an impeding energy barrier or exit energy filter while the impending energy barrier blocks other masses having lower axial energies.
An RF-only mass filter need not require as much precision in the fabrication and assembly of the quadrupole electrodes in order to achieve reasonable resolutions and extended mass range, since the unwanted ion discrimination is assisted by the energy filtering. However, the resolution may still not be as good as that exhibited by the high-precision mass filters. Incorporation of a stopping element on the axis at the exit of the quad results in some improvement in performance but with sacrifice of desired ion transmission. There is thus a need for an improved, low-cost mass filter design.